1. Technical Field
The present invention relates to a sensor for sensing deceleration of a vehicle, and particularly to a sensor for actuating a vehicle occupant restraint such as an air bag.
2. Description of the Prior Art
Co-pending U.S. patent application Ser. No. 500,691, filed Mar. 28, 1990 and assigned to the assignee of the present invention, describes a sensor having a base and a roller movable on a guide surface of the base. The sensor has two independent electrical circuits. Each circuit includes a flexible metallic band which wraps around the roller. Each band has its ends fixed to the base. The bands are under tension and bias the roller into an initial position. Upon vehicle deceleration above a predetermined rate, the roller rolls on the guide surface out of its initial position, causing portions of the bands to engage firing contacts to complete electrical circuits within the sensor. Current flowing through these completed circuits is used to actuate a vehicle occupant restraint system such as an air bag.
In the sensor identified above, throughout the range of motion of the roller along the guide surface of the base, the bands act to bias the roller toward its initial position. A certain amount of force is needed to overcome this biasing effect to move the roller. The amounts of force needed to move the roller toward its firing contacts, at all points in the roller's range of motion along the base, constitute the sensor's force curve. If the sensor's force curve is known, the sensor can be calibrated properly so that the electrical circuits in the sensor will be completed upon experiencing vehicle deceleration of a predetermined amount. Preferably, a large number of sensors with identical known force curves can feasibly be manufactured on an automated assembly line.
It is difficult to meet these objectives using the design of the rolamite sensor identified above. Each band in that rolamite sensor includes an angled connector portion extending between two straight portions. The straight portions are parallel but non-collinear. Because the bands are under tension due to forces applied to the ends of the straight portions of the bands, a twisting moment is produced in the angled connector portion of the band. This twisting moment can result in localized elastic buckling of the band, and the band does not lie flat on the guide surface of the base. If a band is buckled, it may not be possible to predict with accuracy how much force is needed to move the roller at a given point on the surface of the base. Thus proper sensor calibration then becomes more difficult.
To minimize this problem, the connector portions of the bands of the rolamite sensor identified above have been designed with circular openings which receive projecting nubs on the surface of the roller, to retain the bands in position relative to the roller. Because of production tolerances and assembly requirements, the circular openings in the bands which receive the nubs on the roller must be significantly larger than the nubs. Consequently, the bands can still move relative to the roller in the assembled sensor. Thus, the bands are still susceptible to twisting as a result of the tension forces applied to the band ends, and the bands can still buckle.